1. Field of the Invention
The present invention relates to a sheet material conveying device that separates and conveys a plurality of sheet materials one by one from a sheet material housing unit that houses the sheet materials and also relates to an image scanning device and an image forming apparatus including the sheet material conveying device.
2. Description of the Related Art
Image scanning devices included in image forming apparatuses, such as scanners, facsimiles, and copiers are provided with automatic document feeders (hereinafter, referred to as “ADFs”). ADFs sequentially feed sheet documents one by one to an image scanning unit for sequential scanning of image information. When conveying documents, ADFs may possibly convey a plurality of overlapping documents, which is called multi-feed. When multi-feed occurs, a part of the documents sequentially fed to the image scanning unit fails to be scanned, resulting in page missing. To address this, there have been developed ADFs including a multi-feed detecting unit that detects multi-feed occurring in conveyed documents at a multi-feed detection position in a document conveying path.
The ADFs disclosed in Japanese Patent No. 4451724 and Japanese Patent No. 4451723 each include an ultrasonic transmitting unit and an ultrasonic receiving unit at positions facing each other with a sheet overlap sensing position in a document conveying path interposed therebetween and include a sheet overlap sensing device that senses multi-feed using ultrasonic waves. In the sheet overlap sensing device, the ultrasonic transmitting unit transmits ultrasonic waves at a timing when a document is present at the portion sandwiched by the ultrasonic transmitting unit and the ultrasonic receiving unit in the conveying path, and the ultrasonic receiving unit receives the ultrasonic waves. The attenuation amount of ultrasonic waves is different between the case where the ultrasonic waves pass through one document and the case where the ultrasonic waves pass through two or more documents. By checking the attenuation amount of the ultrasonic waves received by the ultrasonic receiving unit with respect to the ultrasonic waves transmitted by the ultrasonic transmitting unit, it is possible to determine whether multi-feed occurs.
The sheet overlap sensing device using ultrasonic waves includes the ultrasonic transmitting unit, the ultrasonic receiving unit, and a sensing signal control unit that controls the ultrasonic transmitting unit and the ultrasonic receiving unit. The sensing signal control unit performs control for transmitting a signal to perform ultrasonic wave transmission to the ultrasonic transmitting unit. In addition, the sensing signal control unit receives a reception signal generated based on the ultrasonic waves received by the ultrasonic receiving unit from the ultrasonic receiving unit and controls a signal to be output based on the received reception signal. The sheet overlap sensing device may possibly make an erroneous determination on multi-feed because of the following failures: a failure that occurs in any one of the ultrasonic transmitting unit, the ultrasonic receiving unit, and the sensing signal control unit; and a failure that occurs in a line connecting the ultrasonic transmitting unit and the sensing signal control unit or a line connecting the ultrasonic receiving unit and the sensing signal control unit. In other words, the sheet overlap sensing device may possibly determine that multi-feed occurs despite no multi-feed occurring, and may also possibly determine that multi-feed does not occur despite multi-feed occurring.
Japanese Patent No. 4451724 describes a configuration that enables the device to make a self-diagnosis on whether the sheet overlap sensing device operates normally or abnormally when the power of the device is turned on. Specifically, let us assume a case where an internal document sensing device configured to sense the presence of a document in the conveying path has detected absence of a document when the power of the device is turned on. In this case, if the output value of the ultrasonic receiving element is smaller than a reference value while the ultrasonic transmitting unit is active, it is determined that the sheet overlap sensing device operates abnormally. Also, in a case where the internal document sensing device has detected absence of a document, if the output value of the ultrasonic receiving unit is larger than the reference value while the ultrasonic transmitting unit is inactive, it is determined that the sheet overlap sensing device operates abnormally. Thus, the device makes a self-diagnosis on whether the sheet overlap sensing device operates normally or abnormally. This can prevent the documents from being conveyed when the sheet overlap sensing device has a failure and may possibly make an erroneous determination on multi-feed.
In the ADF described in Japanese Patent No. 4451724, if the internal document sensing device detects that a document is present in the conveying path when the power of the device is turned on, the ADF notifies a user that the document is present. After the user finishes removing the document, the ADF performs control for checking whether a document is present in the conveying path again. After detecting that no document is present, the ADF performs control for checking whether a failure occurs in the sheet overlap sensing device.
With a document present in the conveying path, such an ADF does not detect a failure occurring in the sheet overlap sensing device before the user finishes removing the document even if the failure has already occurred in the sheet overlap sensing device when the power of the device is turned on. The failure in the sheet overlap sensing device is repaired by a professional repairer called a service person by replacement of parts and connection of lines, for example. If it is detected that a failure occurs in the sheet overlap sensing device after the user finishes removing the document, the following problem occurs. Although the user expects scanning of a document to be enabled immediately by removing the document, the failure in the sheet overlap sensing device is detected after removing the document. As a result, the user needs to ask a service person to come, failing to scan the document immediately, after all.
The problem described above occurs not only in a document conveying device that conveys a document as a sheet material. It may possibly occur in any sheet material conveying device that conveys one sheet material from a sheet housing unit separately from other sheet materials and includes a multi-feed detecting unit configured to check whether a plurality of sheet materials to be conveyed separately do not overlap with one another. Furthermore, the problem occurs not only in a sheet material conveying device including a multi-feed detecting unit using ultrasonic waves. It may possibly occur in any sheet material conveying device that detects occurrence of a failure in the multi-feed detecting unit and whether a sheet material remains in a sheet material conveying path at a predetermined timing, such as a timing of power-on.
In view of the above, there is a need to provide a sheet conveying device that can notify, when a failure occurs in a multi-feed detecting unit with a sheet material present in a conveying path, a user that the failure occurs in the multi-feed detecting unit before causing the user to expect conveyance of a sheet material to be enabled simply by removing the sheet material. There is also a need to provide an image scanning device and an image forming apparatus including such a sheet conveying device.